The Z-1, a metallic girder-like structure, houses radio communications equipment and gyroscopes to keep the ISS steady in space.

"The crew...worked absolutely perfectly together, " said Chuck Shaw, an official with the American space agency, Nasa. "It's a major achievement for this complicated an EVA (extravehicular activity) to go this well."

It was the first of four spacewalks planned for the Discovery crew.

McArthur and Chiao were assisted in their work by Japanese astronaut Koichi Wakata. He moved the men into place on the shuttle's massive mechanical arm.

Solar arrays

The spacewalkers also installed an antenna to the new component, and put a toolbox on the exterior of the station for use in future construction work.

Nasa's impression of the Z-1 Truss sitting atop the Unity module

Next to venture outside the orbiter will be Jeff Wisoff and Mike Lopez-Alegria. They will assist with the installation of a new shuttle docking port. The Pressurized Mating Adapter 3 (PMA3) will be used by the shuttle Endeavour when it arrives at the ISS in early December.

Endeavour will bring up the large solar arrays that need to be hooked on to the Z-1 truss.

Astronauts have never attempted four EVAs before on a space station mission. The most spacewalks conducted on a single shuttle flight is five. They were made during a critical mission to repair to the Hubble Space Telescope.

Nasa's big disappointment is that hardly any of the spacewalking on this mission has been seen on Earth.

Expedition One

An antenna failure aboard Discovery has prevented the crew of seven from beaming down live, continuous video since Thursday.

The astronauts have had to use a slower shuttle antenna to relay staggered snapshots and occasional snippets of video.

Discovery will return to Earth on 22 October. Eight days later, a three-man US-Russian crew, the first residents of the ISS, are due to blast off from the Baikonur cosmodrome in Kazakhstan aboard a Soyuz rocket.

Expedition One, as it is called, will include US astronaut William Shepherd and the Russian cosmonauts Sergei Krikalev and Yuri Gidzenko. They will remain aboard the ISS until February 2001, when a US space shuttle will bring them back to Earth.

The 16-nation space station project is expected to be completed in 2006, but work in the ISS lab will continue until at least 2013.